Overcurrent relays

ABSTRACT

This invention relates to an overcurrent relay in which a voltage is developed proportional to the current in an AC circuit, a two-state switch is responsive to an overcurrent condition in which the monitored voltage periodically exceeds a reference level whereby to assume one state for the duration of these periods during which this level is exceeded, a storage circuit progressively accumulates a voltage from a datum level during each period for which the switch is in its other state, and a timer is operable whenever the switch assumes its one state to actuate an output device after a predetermined delay, the timer being reset by the storage circuit each time its voltage attains a predetermined magnitude. The maximum &#39;&#39;&#39;&#39;overshoot&#39;&#39;&#39;&#39; of the relay, i.e., the effect which could cause the output device to operate and effect protective action even with an unsustained overcurrent, may be contained within a period just in excess of one cycle, or even less, and the pickup/dropoff ratio of the output device can practically approach the optimum value of 100 percent.

Simpson Feb. 22, 1972 [54] OVERCURRENT RELAYS 1 Primary Examiner-JamesD. Trammell 4 [72] Inventor: Michael Charles Stephen Simpson, Stafg g g& Douglas Ke'th M lsegades and ford, England eorge (mg [73] Assignee:The English Electric Company Limited, [57] ABSTRACT London, England This1nvent1on relates to an overcurrent relay 1n which a volt- Filed!- Allg.5, 1968 age is developed proportional to the current in an AC circuit, atwo-state switch is responsive to an overeurrent condition in [21] Appl't 750,064 which the monitored voltage periodically exceeds a referencelevel whereby to assume one state for the duration of these ForeignApplication D periods during which this level is exceeded. a storagecircuit Aug 3, 1967 Great Britain "35'634/67 progressively accumulates avoltage from a datum level during each period for which the switch is inits other state, and a [521 us. Cl. ..317/36 TD, 317/38, 317/142 timeris Operable Whenever the Switch assumes State 51 Int. (:1. .J-lOlh 47/18actuate an Output device after a predetermined y. th [58] Field ofSearch..3l7/36 TD, 142, 38 timer being reset y the Storage circuit each timeits voltage p v attains a predetermined magnitude. [561 References CitedThe maximum overshoot of the relay, i.e., the effect which UNITED STATESPATENTS could cause the output device to operate and efi'ect protectiveaction even with an unsustained overcurrent, may be con- 3,127,5423/1964 Riebs ..3l7/36 X g i d ithin a iod ju t in excess of one cycle,or even less, 3,262,017 7/ 1966 As henden et X and the pickup/dropofiratio of the output device can practi- 3,3 l Pnce et a] 1 ally approachthe optimum value of percent 3,334,272 8/1967 Lipnitz ..3l7/36 l 1Claims, 8 Drawing Figures PATENTEUFEB22 I972 3,644,789

sum 3 [IF 4 FIG.3

OVERCURRENT RELAYS This invention relates to a protective relayresponsive to an overcurrent in an AC circuit, and in particular relatesto definite time overcurrent relays, i.e., relays in which apredetermined period is allowed to elapse between the instants at whichan overcurrent is sensed and protective action taken.

According to the present invention there is provided an overcurrentrelay comprising monitoring means for developing a voltage proportionalto the current in an AC circuit, a two-state switch responsive to anovercurrent condition in which the monitored voltage periodicallyexceeds a reference level whereby to assume one state for the durationof these periods during which this level is exceeded, a storage circuitoperable progressively to accumulate a voltage from datum level duringeach period for which the switch is in its other state, and a timingcircuit operable whenever said switch assumes its one state to actuatean output device after a predetermined delay and which is reset by thestorage circuit each time its voltage attains a predetermined magnitude,the

, .arrangement being such that the stored voltage attains said magnitudein a period not less than that between succeeding instants at which themonitored voltage exceeds the reference level during an overcurrent.

Preferably, an auxiliary circuit is provided for instantaneouslyoperating the output device, bypassing the timing circuit, in responseto a substantial voltage being developed by the monitoring means andwhich is indicative of an overcurrent of excessive magnitude.

The relay may readily be connected for operation on single phase ormultiphase systems and may also include earth fault protection, themonitoring means, e.g., current, or voltage, transformers and associateddiodes, being designed appropriately. Furthermore, the relay may be madevoltage sensitive, that is, the aforesaid reference level may vary indirect proportion to the voltage subsisting in the AC circuit.

A relay according to this invention provides in a simple and I efficientmanner adequate protection against excessive currents damagingelectrical apparatus and systems and is yet relatively insensitive totransient overcurrents. The maximum degree of overshoot i.e., the effectwhich could cause the output device to operate and effect protectiveaction even with an unsustained overcurrent, may conveniently becontained within a period just in excess of one cycle or even lessdepending on the complexity of the monitoring means, i.e., whetherhalf-wave or full-wave diode bridges are employed.

The pickup/dropoff ratio of the output device can practically approachthe optimum value of 100 percent and this value can be consistentbetween singlephase and balanced multiphase faults depending on theconstruction and loading of the transformers in the monitoring means. Inaddition, the design of the timing circuit is such as to impose alow-standing direct current drain on the relay and the complexity of thesettings is minimized by circuit design; this is particularly true inrespect of the auxiliary (instantaneous) circuit where its threshold ofoperation is determined by a single variable resistor which mayconveniently be calibrated directly in amperes.

In order that the invention may be fully understood, one embodimentthereof will now be described with reference to the accompanyingdrawings, in which:

FIG. 1 is a block diagram of an overcurrent relay according to thisinvention;

P10. 2 is a detailed circuit diagram of the starter, the timer and theinstantaneous highset" unit in the above relay;

FIGS. 3A to 3D are waveform diagrams illustrating the mode of operationof the starter for one phase of a three-phase system;

FIG. 4 shows an alternative input circuit to that employed in FIG. 2;and

FIG. 5 shows a modified input circuit for rendering the relay voltagesensitive.

With reference to the block diagram, the relay comprises an overcurrentstarter 4 which is connected to receive a signal representative of theinstantaneous current monitored from the protected section of, say, apower line and is operative to actuate a timer 5 in response to anovercurrent. This timer is activated continuously in the event of theovercurrent persisting over successive cycles and an output relay 6,e.g., a hinged armature e/m relay, is then subsequently operative toisolate the line in dependence on the delay occasioned by the timer.

In addition, an instantaneous highset" unit 7 is provided for protectingthe line in the event of an abnormal overcurrent, this unit functioningto bypass the timer and actuate directly the output relay 6.

Referring now to FIG. 2, the overcurrent starter 4, the timer 5 and theinstantaneous unit 7 are shown in more detail, the system being designedto detect overcurrents occurring in a three-phase power supply line.

In particular, current transformers CTl-CT3 in the overcurrent starterare associated with each phase and develop across their correspondingsecondary resistors R1, R2-RS, R6 a voltage proportional to the inputcurrent through these resistors. Three like poled diodes Dl-D3 areconnected together from the secondaries to a potentiometer RVI whichlies in common across these circuits and the wiper arm of thispotentiometer is connected to a zener diode ZDl in series with tworesistors R7, R8. The junction between these resistors is connected tothe base electrode of a transistor TRl having a limiting diode D4connected across its base-emitter junction and a capacitor C1 connectedacross its collectoremitter path. The collector supply potential forthis transistor is applied through a resistor R10 and the collectoroutput is applied to the base of a further transistor TR2. This lattertransistor has a variable resistor RV2 in its collector circuit and itsoutput actuates the timer 5. i

In particular, the timer comprises a capacitor C2 connected directlyacross the collector-emitter path of the transistor TR2, the output fromthis transistor being applied to the base of a transistor TR3. Theemitter circuit of TR3 includes a diode D9 which is connected to thejunction between two voltage-regulating zener diodes ZD2, ZD3 which, inturn, are series connected across the supply with a diode D10. Aresistor R12 is connected in the collector circuit of this transistorand its output actuates the relay 6 (FIG. 1).

With regard to the instantaneous unit, three like poled diodes D6D8 arerespectively connected to the junctions of the resistors R1, R2-R5, R6and are coupled together to a v smoothing capacitor C4, thepart-smoothed output being applied to the base electrode of a transistorTR4 through a resistor R13 of a divider chain R13, R14. A protectivediode D12 is connected across the base-emitter junctions of thistransistor and the emitter is held at a potential determined by apotentiometer RV3 and a series resistor R15 connected to the supplyline. The output from the transistor TR4 actuates the relay 6 directly.

In operation, under normal load conditions transistor TR2 is biassed-ondrawing base current through R10 with capacitor C1 charged, andtransistor TRl is off, the voltage on the wiper arm of RVI being lessthan the zener voltage V of the diode ZDl. In the event of anovercurrent now occurring in any one phase at, say, instant 1' (FIG. 3A)the following positive half cycle which is conducted by the appropriatediode Dl-D3 breaks down the zener diode over the peak of its excursionso that a current pulse (FIG. 3B) flows into the base of transistor TRl,turning it on. With TR] on, the capacitor C1 is discharged (FIG. 3C)causing the base of TR2 to go negative and thus biassing it off. Thecollector voltage of TR2 then rises with a time constant determined byRV2, C2 (FIG. 3D) in the timer circuit and in the event of the voltageacross C2 exceeding the zener voltage of ZD3 transistor TR3 conducts andactuates the output relay.

The transistor TRl in the starter is on for the duration of the periodfor which the zener diode ZDl is conductive, and following this itscollector voltage rises (FIG. 3C) at a rate determined by the timeconstant R10, Cl. Should the overcurrent persist over the next inputcycle then transistor TRl is rendered conductive again before itscollector voltage exceeds the emitter voltage volts) of transistor TR2so, that this transistor is still maintained in the off-state and thetimer continues to operate.

If, as in the example given, the overcurrent ceases at an instant t"then the capacitor C1 is permitted to continue charging and transistorTR2 is ultimately switched-on discharging the capacitor C2 and arrestingthe timer at the instant t' (F IG. 3D). The overshoot or reset time inthis instance, i.e., the time for which the timer continues to operatefollowing the cessation of the overcurrent and perhaps activate theoutput relay, is given by the period between t" and t and it will beevident that this is governed by R10, Cl and, at its-maximum, need onlyjust exceed .the duration of one cycle irrespective ofthe magnitude ofthe overcurrent to which the starter is operable to respond, as set byRVl.

With abnormal and excessive overcurrents however, the instantaneous unit7 is designed to respond immediately in dependence on the setting ofRV3. In particular, the voltages developed across R2, R4 and R6 arehalf-wave rectified by the appropriate diodes D6-D8 and smoothed beforeapplication to the base of transistor TR4. If at any instant the basevoltage on this transistor exceeds the emitter voltage, determined bythe wiper arm of RV3, then this transistor will conduct and directlyactivate the output relay 6, bypassing the timer unit so as immediatelyto trip-out the protected powerline.

Should the overcurrent affect more than one phase then the action of thecircuit will be similar except that there will be more current impulses(FIG. 38) with a consequent decrease in the intermediate amplitudes ofthe voltage across C1 (FIG. 3C). The overshoot time will be unaffected,but this may conveniently be reduced by employing full-wave rectifierbridges instead of the half wave diodes D1-D3 the current impulses thenoccurring with twice the regularity shown; in this event the timeconstant R10, C1 can be halved resulting in a reduction by half in theovershoot time. A similar reduction will occur in the instantaneousoperating time with the use of fullwave rectifiers for the diodes D6-D8.

Although the invention has been described above in relation toprotection of a three phase power supply line it is to be understoodthat the circuit is equally applicable for protecting other systems orapparatus, e.g., generator-transformer equipment. Furthermore, the inputcircuit may readily be simplified for protecting single-phase systemsand protection may also be afforded for two-phase and earth faultoperation of a threephase system. In this latter instance the inputcircuit would be modified as shown in FIG. 4. In particular, in thisFigure like components to those shown in FIG. 2 have been accorded thesame reference numerals and the only changes involve the substitution ofan earth fault current transformer CT4 for the phase transformer CT3,the provision of a separate variable resistor RV4 across the secondaryof CT4 and the use of two isolating diodes D13, D14 in the leads fromRVl and RV4.

A further modification involves making the relay voltage sensitive. inthis instance the input circuit is modified as shown in F l6. 5, thezener diode ZDl (shown dotted) being replaced by a circuit for providinga bias" voltage proportional to the voltage V subsisting in the ACsystem protected, namely, a full-wave diode bridge rectifier DB1 and asmoothing circuit R16, C5. Transistor TRl thus turns on only when theinstantaneous value of the current-dependent input voltage exceeds thisbias" voltage.

Although reference has been made throughout to the use of currenttransformers in the input circuit, voltage transformers mayalternatively be used if the input quantity monitored is voltage.

I claim:

1. A definite time protective relay comprising;

monitoring means for monitoring at least one alternating quantity in anAC circuit to be protected and producing a peak excess signal whenever amonitored quantity exceeds a predetermined level; a first delay circuitfed by the peak excess signals and which produces an output for theduration of and for a predetermined time (in excess of the maximuminterval between successive peak excess signals due to a sustainedfault) after the end of an input signal; and

a second delay circuit fed with the output of the first delay circuitand which produces an output indicating a fault if the input to itexceeds a predetermined duration.

2. A definite time protective relay according to claim 1, wherein thefirst delay circuit includes a capacitor which is charged to a firststate by an input signal and towards a second state when no input signalis present, the circuit producing an output signal when the capacitor isnot in the first state.

3. A definite time protective relay according to claim 2, wherein thefirst delay circuit includes a transistor which is turned on by an inputsignal and is connected to charge the capacitor to the first state whenturned on.

4. A definite time protective relay according to claim 1, wherein thesecond delay circuit includes a capacitor which is normally held in afirst state and is charged towards a second state while an input signalis present, the circuit producing an output signal when the capacitorreaches the second state.

5. A definite time protective relay according to claim 4, wherein thesecond delay circuit includes a transistor which is turned off by aninput signal and is connected to charge the capacitor to the first statewhen turned on.

6. A definite time protective relay according to claim 5, wherein thesecond delay circuit includes a voltage-responsive element subjected tothe voltage on the capacitor in the second delay circuit and adapted tobreak down and produce an output signal in response to this voltageattaining a preset level.

7. A definite time protective relay according to claim 1, including alsoan auxiliary circuit fed by the monitoring means and operable to producean output signal indicating a fault substantially instantaneously if themonitored quantity is of a magnitude indicating an excessive faultcondition.

8. A definite time protective relay according to claim 1, including:

a current transformer having a primary winding connected in the ACcircuit and a secondary winding;

rectifying means connected to the secondary winding of the transformerfor producing a rectified waveform of the derived voltage; and

a voltage-responsive element fed by the rectifying means and adapted tobreakdown and produce a peak excess signal in response to the rectifiedwaveform exceeding the said predetermined level.

9. A definite time protective relay according to claim 1, including:

a current transformer having a primary winding connected in the ACcircuit and a secondary winding;

rectifying means connected to the secondary winding of the transformerfor producing a rectified waveform of the derived voltage; and

a sensing circuit fed by the rectifying means and operable to develop aDC bias voltage, proportional to the voltage subsisting in the ACcircuit, which serves as the said predetermined level, and to produce apeak excess signal in response to the rectified waveform exceeding thesaid predetermined level.

10. A definite time protective relay according to claim 8, adapted formultiphase operation, the current transformers and rectifying meanscorresponding in number to the number of phases and being connected incommon to the voltage responsive element.

11. The invention according to claim 1, wherein said monitoring meansproduces a peak excess signal whenever a monitored quantityinstantaneously exceeds a predetermined level.

1. A definite time protective relay comprising; monitoring means formonitoring at least one alternating quantity in an AC circuit to beprotected and producing a peak excess signal whenever a monitoredquantity exceeds a predetermined level; a first delay circuit fed by thepeak excess signals and which produces an output for the duration of andfor a predetermined time (in excess of the maximum interval betweensuccessive peak excess signals due to a sustained fault) after the endof an input signal; and a second delay circuit fed with the output ofthe first delay circuit and which produces an output indicating a faultif the input to it exceeds a predetermined duration.
 2. A definite timeprotective relay according to claim 1, wherein the first delay circuitincludes a capacitor which is charged to a first state by an inputsignal and towards a second state when no input signal is present, thecircuit producing an output signal when the capacitor is not in thefirst state.
 3. A definite time protective relay according to claim 2,wherein the first delay circuit includes a transistor which is turned onby an input signal and is connected to charge the capacitor to the firststate when turned on.
 4. A definite time protective relay according toclaim 1, wherein the second delay circuit includes a capacitor which isnormally held in a first state and is charged towards a second statewhile an input signal is present, the circuit producing an output signalwhen the capacitor reaches the second state.
 5. A definite timeprotective relay according to claim 4, wherein the second delay circuitincludes a transistor which is turned off by an input signal and isconnected to charge the capacitor to the first state when turned on. 6.A definite time protective relay according to claim 5, wherein thesecOnd delay circuit includes a voltage-responsive element subjected tothe voltage on the capacitor in the second delay circuit and adapted tobreak down and produce an output signal in response to this voltageattaining a preset level.
 7. A definite time protective relay accordingto claim 1, including also an auxiliary circuit fed by the monitoringmeans and operable to produce an output signal indicating a faultsubstantially instantaneously if the monitored quantity is of amagnitude indicating an excessive fault condition.
 8. A definite timeprotective relay according to claim 1, including: a current transformerhaving a primary winding connected in the AC circuit and a secondarywinding; rectifying means connected to the secondary winding of thetransformer for producing a rectified waveform of the derived voltage;and a voltage-responsive element fed by the rectifying means and adaptedto breakdown and produce a peak excess signal in response to therectified waveform exceeding the said predetermined level.
 9. A definitetime protective relay according to claim 1, including: a currenttransformer having a primary winding connected in the AC circuit and asecondary winding; rectifying means connected to the secondary windingof the transformer for producing a rectified waveform of the derivedvoltage; and a sensing circuit fed by the rectifying means and operableto develop a DC bias voltage, proportional to the voltage subsisting inthe AC circuit, which serves as the said predetermined level, and toproduce a peak excess signal in response to the rectified waveformexceeding the said predetermined level.
 10. A definite time protectiverelay according to claim 8, adapted for multiphase operation, thecurrent transformers and rectifying means corresponding in number to thenumber of phases and being connected in common to the voltage responsiveelement.
 11. The invention according to claim 1, wherein said monitoringmeans produces a peak excess signal whenever a monitored quantityinstantaneously exceeds a predetermined level.